Meteo 3: Chapter 16 Winter Weather Read pages , ,

Snow  Almost all precipitation begins as snow in clouds as long as ice nuclei are present (think Bergeron)  The temperature structure of the atmosphere determines the precipitation type at the ground  Snowflakes can be a single ice crystal or several ice crystals stuck together  Supercooled water: Water existing in its liquid state below 32°F

Snow terminology  Snow squall: Heavy, brief burst of snow  Snow shower: Moderate, brief burst of snow  Snow flurries: Very light, brief snow shower with no accumulation –Above three categories are analogous to “convective” precipitation (cumuliform clouds)  Long-duration (stratiform) snows categorized based upon visibility  Blizzard: At least 3 consecutive hours with winds >= 35 mph & falling/blowing snow reducing visibility to = 35 mph & falling/blowing snow reducing visibility to <.25 mi –Ground blizzard if no falling snow

Can it snow with ground temperatures above freezing?  YES!!! –It takes time for falling snowflakes to melt –An above freezing layer ~1000 ft thick near ground needed for melting –Can have snow with surface temperatures at 40F

Sleet (Ice Pellets)  “Calling sleet hail is tantamount to comparing roast beef to Swiss cheese”

Freezing Rain  Leaves objects coated in ice  Dangerous…can down wires, trees, can cause auto accidents See CD Animation

Forecasting Precipitation Type: mb Thickness (5400 m “critical” thickness)

Forecasting Precipitation Type: 850 mb Temperature

A Note on Forecasting Precipitation Type  850 mb temperature is a better indicator of precipitation type because it’s a direct measure of temperature in the lower troposphere  5400 m mb critical thickness represents 50%-50% probability of liquid versus frozen precipitation

Alberta Clippers- Mostly light snow due to limited moisture, fast speed

Jet stream pattern leading to east coast cold and snow

Ingredients for Northeast Winter Storms  Sharp upper-level trough…leads to upper-air DIV and pressure lowering  “Bomb” if low deepens >= 12 mb in 24 hours  Arctic high pressure center in SE Canada enhances NE wind, which funnels in cold air & moisture for overrunning

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More Nor’ Easter Facts  Named as such because of strong NE winds associated in advance of low  Most frequent between December & March  Besides causing heavy snow, ice, and rain, can lead to coastal flooding and beach erosion

Lake-Effect Snow

 Primarily generated in late fall-mid winter as cold air moves over warm Great Lake waters –Cold air heated and moistened as it passes over lakes  Snowfall can be extreme (2-4 inches per hour, several feet over a day or two)  Some enhancement from frictional convergence at lake shore  Enhanced when warmed, moistened air encounters higher terrain…forced lifting = more snow –Heaviest snow usually occurs slightly inland from lake shore

More on Lake-Effect Snow  The longer the wind trajectory over the lake, the more heating and moistening of arctic air occurs, the heavier the snow  Falls in narrow bands aligned with the wind direction –Bands relatively narrow => forecasting challenge

Ocean-Effect Snow

Winter Weather Forecasting is Tough!  Precipitation type?  Enhanced bands of heavy snowfall  Imperfect forecast models predict precipitation in terms of liquid equivalent –Converting that to snowfall is challenging –General rule: “ 10 inches snow = 1 inch water” –At temperatures well below freezing, this ratio can be significantly higher  Difficult to measure snow, as snow compacts on the ground  Snow/no snow boundary can be very sharp

Image taken by the MODIS instrument on NASA's Terra satellite at 11:40 CST December 26, 2004.MODIS

Wind-Chill Temperature  Air flowing by the skin moves a thin layer of warm air in contact with the skin away  The wind-chill is an apparent temperature, describing the enhanced sensation of cold as a result of the wind –Frostbite can occur on exposed skin in 30 minutes or less when wind-chill values are less than -20°F

Wind Chill Temperature